The present invention relates to a illuminated airfield signs of the type used for marking ground traffic routes on runways and taxiways. The invention is more particularly concerned with an improved airport guidance sign having a curved face, and which is capable of with-standing the prescribed wind load tests and at the same time has even illumination across its legend panel(s).
Illuminated guidance signs are commonly used at airports to identify runways and taxiways and provide ground route marking information for air crews. These guidance signs have transparent or translucent legend panels supported in a frame or case, with the case being supported on legs that provide a minimum ground clearance.
Guidance signs with curved or bowed faces have become a preferred design at many airports, because the design of the curved face inherently sheds jet blast better than a standard flat face, and because the design provides balanced lighting and even illumination without complex internal mechanism.
The standards for airport guidance signs in the United States are established by the Federal Aviation Administration, and in particular the Specification for Runway and Taxiway Signs, as specified in Advisory Circular 150/5345-44G, which was issued Jul. 8, 2004. This specification requires a wind load and frangibility test (paragraph 4.1.1.2), in which a simulated wind load, i.e., a static load of 0.9 psi is uniformly applied to the entire face of the legend panel for ten minutes, and after which both the legend panels and the support legs are inspected for damage. For the sign to pass this test, there can be no evidence of breakage or distortion. Then the static load applied to the sign legend panel is increased until the frangible connector of the support leg gives way, and this frangible point failure must occur before the legend panel loading reaches a static pressure load of 1.3 psi for classification as a Mode 2 sign or 2.0 psi for classification as a Mode 3 sign. Again the sign is inspected for damage and there can be no distortion of breakage of the legend panel.
The frangible connector is intended to provide a safe failure mode for the signs so that the danger is minimized, for example, if an aircraft rolls into or over a guidance sign.
It has recently been discovered that the guidance signs with curved legend panels do not always withstand the static loads as prescribed in this test, and may fail due to flexure and distortion of the curved legend panels under the simulated wind load. Accordingly, it has become necessary to find ways to avoid buckling or distortion that arises under the pressures of the above test specification. This includes adding material to the case or frame to combat buckling, or adding internal ties and supports. However, these approaches can create undesirable shadows and bright spots in the legend panels, and can affect their readability adversely.